These are now combined into an inaddr.[ch], since their purpose is
to implement functions for struct in_addr and struct in6_addr.
The definitions for in6_addr and its allies are now in a separate
header, inaddr_st.h.
Closes ticket 26532.
This code was in compat_threads, since it was _used_ for efficiently
notifying the main libevent thread from another thread. But in
spite of its usage, it's fundamentally a part of the network code.
